During the last funding period, we made the unexpected observation that memory CD8 T cells that developed after secondary stimulation exhibited a myriad of differences in phenotype and function from primary memory CD8 T cells(Jabbari and Harty, J. Exp. Med., 2006). More recently, we have observed that primary and secondary memory CD4 T cells also exhibit differences, however, with respect to one key molecule (CD62L, which controls homing of T cell to lymph nodes) primary and secondary memory CD4 T cells display the exact opposite pattern of expression to that seen on primary and secondary memory CD8 T cells. The fact that the immune system regulates access to lymph nodes differentially, not only between primary and secondary memory CD4 and CD8 populations, but also between CD4 and CD8 memory T cells, suggests important consequences to the overall function of the immune response. Additionally, although most current human vaccines employ booster immunizations and will thus generate secondary memory T cell populations, there are only a few studies besides ours on secondary memory CD8 T cells and essentially no published information on the characteristics of secondary CD4 T cell memory. Given the clear relevance of secondary memory to human vaccines that employ booster immunizations, we decided to focus (and re-title) this competitive renewal on "Regulation of primary and secondary CD4 and CD8 T cell memory" to address these knowledge gaps. This competitive renewal remains consistent with the long-term goals associated with the previous funding periods of this grant-to understand how memory T cells are generated and provide immunity to intracellular pathogens. Aim 1. Define the characteristics of primary versus secondary memory CD4 T cells in response to infection. Aim 2. Determine the mechanisms resulting in maintenance of secondary memory CD4 and CD8 T cells. Aim 3. Evaluate functional differences between primary versus secondary memory CD4 T cells and CD8 T cells in response to pathogens with diverse characteristics. Aim 4. Determine the molecular mechanisms that regulate the opposite patterns of CD62L expression in primary and secondary memory CD4 versus CD8 T cells. PUBLIC HEALTH RELEVANCE Booster immunizations are often used to enhance protective T cell numbers and are a common feature of vaccines used to protect humans against infectious disease. Our preliminary data generated during the last funding period shows that boosted (2[unreadable] memory) T cells are quite different than 1[unreadable] memory T cells. The goal of this proposal, to fully characterize the functional and molecular consequences imposed on T cell populations by multiple antigen exposures, will be significant in understanding how best to generate protective immunity by vaccination.